In prior art polymers consisting of repeating thiophene units have been reported to show good performance as charge transporting materials in FET applications. For example, regioregular poly(3-alkyl)thiophene for example has demonstrated one of the highest recorded mobilities to date for a polymer (Sirringhaus et al., Science, 1998, 280, p 1741). Also, polythiophene analogues as disclosed for example in EP 1 327 646 A1, EP 1 327 647 A1, EP 1 329 474 A1 or EP 1 329 475 A1 containing different numbers and regioisomers of alkyl-thiophenes exhibit reasonable charge carrier mobilities.
This performance is thought to be due to two factors. Firstly the arrangement of the alkyl side-chains on the polymer backbone allows the polymers to self-organise into well-ordered structures on coating from solution. This facilitates the hopping mechanisms that dominate charge transport. Secondly the presence of sulfur atoms in the polymer backbone has been shown to be beneficial to charge transport. The exact mechanism is not known, but it is speculated that interaction of the sulfur d-orbitals on adjacent polymer chains facilitates the charge hopping mechanism.
However, the polymers disclosed in the above prior art documents do only show charge carrier mobilities of not more than 0.1 cm2V−1s−1. Also, the materials of prior art often show only limited solubility which is a disadvantage when processing the polymers for the manufacture of semiconductor devices like thin film transistors (TFT) or field effect transistors (FET).
Therefore, further enhancement of the charge mobility and solubility of organic polymers is desired in order to enable transistor performance.
It was an aim of the present invention to provide new organic materials for use as semiconductors or charge transport materials, which are easy to synthesise, have high charge mobility and good processibility. Especially the materials should be easily processible to form thin and large-area films for use in semiconductor devices. Also, the materials should be oxidatively stable, but retain or even improve the desired electrical properties.
The inventors of the present invention have found that materials based on thieno[3,2-b]thiophene (1) (TT)
in particular monomers comprising a mesogenic core with a TT group and one or more polymerisable groups attached thereto, show improved charge carrier mobility whilst maintaining desirable solution processable properties.
In prior art there are no reported examples of molecules containing TT groups that exhibit LC behaviour. Nakayama et al., Tetrahedron 1996, 52, p 471 report dimers, trimers and tetramers of 3,6-dimethyl-TT as shown below.

However, these materials are neither designed nor reported to exhibit LC behaviour, and additionally are reported to exhibit low solubility. No electronic behaviour is reported. High mobility is not expected with these materials because of the steric twisting, caused by the interaction of the adjacent methyl groups in the 3 and 6 positions.
Zhang and Matzger, J. Org. Chem. 2003, 68, p 9813-0815, disclose TT compounds as shown below.

These molecules are unsubstituted and do not exhibit LC behaviour. No electrical data is reported.
WO 99/12989 discloses oligomers and polymers comprising two or more fused thiophene rings which may be substituted or unsubstituted for use in TFTs and FETs. However, there is no specific disclosure of mesogenic TT monomers with polymerisable terminal groups or their preparation.
EP 1 275 650 A2 discloses compounds comprising one or more fused thiophene rings and polymerisable groups, but does not explicitly disclose compounds according to the present invention.
Thus, another aim of the invention was to provide thieno[3,2-b]thiophene (TT) materials that are more easily processible in the manufacture of semiconductor devices, have higher stability and allow easier synthesis also at large scale compared to TT materials of prior art.
It was found that the above aims can be achieved by providing compounds according to the present invention.